Roach, G.I.D.

The silica in hydrate and smelter-grade alumina occurs in three main forms: as discrete particles derived from the mud residue, as silica co-precipitated in some form within the hydrate and as desilication product (DSP). For plants utilising sand filters the higher silica values are primarily from unfiltered mud residue. The silica derived from DSP is the cause of most product silica variations for refineries which utilise pre-digestion desilication. Dilute acid leaching has been used to quantify the DSP silica, both that on the surface of the hydrate particles and within the particles. The fine particles contain higher silica values because of a higher proportion of surface DSP silica. The precipitation of DSP under typical hydrate precipitation conditions using hydrate seed was studied. The amount of DSP silica incorporated into hydrate is determined not only by kinetic factors such as silica supersaturation, precipitation temperature and the surface area of the hydrate, but also hydrate morphology. Methods to minimise DSP precipitation which lead to a reduction of the variability of silica in product alumina are discussed.